Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent
  • C22C21/04—Modified aluminium-silicon alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C32/00—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C49/00—Alloys containing metallic or non-metallic fibres or filaments
  • C22C49/02—Alloys containing metallic or non-metallic fibres or filaments characterised by the matrix material
  • C22C49/04—Light metals
  • C22C49/06—Aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/043—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with silicon as the next major constituent

Definitions

• This invention relates generally to aluminum-silicon (Al-Si) alloys. It relates particularly to a high strength Al-Si based alloy suitable for high temperature applications for cast components such as pistons, cylinder heads, cylinder liners, connecting rods, turbo chargers, impellers, actuators, brake calipers and brake rotors.
• Al-Si alloys are most versatile materials, comprising 85% to 90% of the total aluminum cast parts produced for the automotive industry. Depending on the Si concentration in weight percent
• the Al-Si alloy systems fall into three major categories: hypoeutectic ( ⁇ 12% Si), eutectic (12- 13% Si) and hypereutectic (14-25% Si).
• hypoeutectic ⁇ 12% Si
• eutectic (12- 13% Si
• hypereutectic 14-25% Si
• most prior alloys are not suitable for high temperature applications because their mechanical properties, such as tensile strength and fatigue strength, are not as high as desired in the temperature range of 500 °F - 700 °F.
• many of the Al-Si cast alloys are intended for applications at temperatures of no higher than about 450 °F. Above this temperature, the major alloy strengthening phases such as the ⁇ ' (AI 2 CU) and S' (AkCuMg) phase will become unstable, rapidly coarsen and dissolve, resulting in an alloy having an undesirable microstructure for high temperature applications.
• Such an alloy has little or no practical application at elevated temperatures because, when the ⁇ ' and S' become unstable, the alloy lacks the lattice coherency between the aluminum solid solution lattice and the strengthening particles lattice parameters.
• a large mismatch in lattice coherency contributes to an undesirable microstructure that can not maintain excellent mechanical properties at elevated temperatures.
• MMC Metal Matrix Composites
• U.S. Patent No. 5,620,791 relates to an MMC comprising an Al-Si based alloy with an embedded a ceramic filler material to form a brake rotor for high temperature applications.
• An attempt to improve the high temperature strengths of Al-Si alloys was also carried out by R. Bowles, who has used ceramic fibers to improve tensile strength of an Al-Si 332.0 alloy, in a paper entitled, "Metal Matrix Composites Aid Piston Manufacture,” Manufacturing Engineering.
• CMC Ceramic Matrix Composites
• W. Kowbel has described the use of non-metallic carbon-carbon material for making pistons to operate at high temperatures in a paper titled, "Application of Net-Shape Molded Carbon-Carbon Composites in IC engines," Journal of Advanced Materials, July 1996.
• manufacturing costs employing these MMC and CMC technologies are substantially higher than those using conventional Al-Si casting, which has hampered their ability to be priced competitively with Al-Si alloys in mass production for high temperature internal combustion engine parts and brake applications.
• an Al-Si alloy containing dispersion of particles having LI 2 crystal structure in the aluminum matrix is presented.
• the alloy is processed using low cost casting techniques such as permanent mold, sand casting or die casting.
• the alloy of the present invention maintains a much higher strength at elevated temperatures (500 °F and above) than prior art alloys, due to a unique chemistry and microstructure formulation.
• the methods for strengthening the alloy in the present invention include: 1) Maximizing the formation of major strengthening ⁇ ' and S' phase in the alloy, with chemical composition given as Al 2 Cu, Al 2 CuMg, respectively.
• both the aluminum solid solution matrix and the particles of A1 3 X compounds should have similar face-centered-cubic (FCC) crystal structures, and will be coherent because their respective lattice parameters and dimensions are closely matched.
• FCC face-centered-cubic
• FIG. 1 is a diagram illustrating a coherent particle that has similar lattice parameters and crystal structure relationship with the surrounding aluminum matrix atoms.
• FIG. 2 is a diagram illustrating a non-coherent particle having no crystal structural relationship with the surrounding aluminum matrix atoms. Such an alloy has little or no practical application at elevated temperatures.
• FIG. 3 is an electron micrograph showing the size and shape of the alloy ⁇ ' and S' coherent phases for prior art alloys as observed at room temperature.
• FIG. 4 is an electron micrograph showing the size, shape and the amount of the alloy strengthening ⁇ ' and S' coherent phases for the alloy of this invention as observed at room temperature.
• FIG. 5 is an electron micrograph showing the transformation of ⁇ ' and S' coherent phase, as observed in FIG 3, into the undesirable ⁇ and S noncoherent phases for the prior art alloys after they have been exposed to 600 °F for 100 hours.
• the ⁇ and S phases are noncoherent because they become unstable rapidly coarsen and dissolve, resulting in an alloy which has an undesirable microstructure for high temperature applications.
• FIG. 6 is an electron micrograph showing the highly stable ⁇ ' and S' coherent phases for the alloy of this invention after it has been exposed to 600 °F for 100 hours. Unlike the prior art, the alloy of this invention still retains the ⁇ ' and S' coherent phases, which are a desirable microstructure for high temperature applications.
• FIG. 7 is a chart showing a comparison of an alloy according to the present invention with three well-known prior art alloys (332, 390 and 413).
• the chart compares the ultimate tensile strengths (tested at 500 °F, 600 °F and 700 °F), after exposure of all test specimens to a temperature of 500 °F, 600 °F, 700 °F for 100 hours, respectively.
• the present invention includes detailed compositional, microstructure and processing aspects through conventional casting processes.
• the Al-Si alloy of the present invention is marked by an ability to perform in cast form, which is suitable for elevated temperature applications. It is comprised of the following elements, in weight percent:
• Silicon gives the alloy a high elastic modulus and low thermal coefficient of expansion.
• the addition of silicon is essential in order to improve the fluidity of the molten aluminum to enhance the castability of the Al-Si alloy according to the present invention.
• the alloy exhibits excellent surface hardness and wear resistance properties.
• Copper co-exists with magnesium and forms a solid solution in the aluminum matrix to give the alloy age-hardening properties, thereby improving the high temperature strength. Copper also forms the ⁇ ' phase compound (Al 2 Cu), and is the most potent strengthening element in this new alloy.
• the enhanced high strength at high temperatures is affected if the copper wt% level is not adhered to.
• the alloy strength can only be maximized effectively by the simultaneous formation for both of the ⁇ ' (Al 2 Cu) and S' (A ⁇ CuMg) metallic compounds, using proper addition of magnesium into the alloy relative to the elements of copper and silicon.
• the alloy composition was specifically formulated with copper-to-magnesium (Cu/Mg) ratios ranging from 4 to 15, with a minimum value for magnesium of no less than 0.5 wt%.
• Cu/Mg copper-to-magnesium
• Si/Mg silicon-to- magnesium
• the unique Cu:Mg ratio greatly enhances the chemical reactions among aluminum (Al), copper (Cu) and magnesium (Mg) atoms.
• FIG.4 is an electron micrograph showing the formation, shape, size and amount of the alloy strengthening ⁇ ' and S' coherent phases for the alloy of this invention as observed at room temperature.
• both the aluminum solid solution matrix and the particles of A1 X compounds have similar face-centered-cubic (FCC) crystal structures, and are coherent because their respective lattice parameters and dimensions are closely matched.
• FIG. 1 is a diagram illustrating a coherent particle that has similar lattice parameters and crystal structure relationship with the surrounding aluminum matrix atoms.
• Zirconium also forms a solid solution in the matrix to a small amount, thus enhancing the formation of GP (Guinier-Preston) zones, which are the Cu-Mg rich regions, and the ⁇ ' phase in the Al-Cu-Mg system to improve the age-hardening properties.
• GP Guard-Preston
• the stable ⁇ ' Al 2 Cu
• Ti, V, and Zr elements in the alloy cannot be discounted.
• High temperature strength characteristics of the alloy of this invention are detrimentally affected if Ti, V, and Zr are not used simultaneously in the proper amount for forming Al (Ti, V, Zr) precipitates.
• FIG. 6 is an electron micrograph showing the highly stable ⁇ ' and S' coherent phases for the alloy of this invention after it has been exposed to temperatures of 600 °F for 100 hours. Unlike alloys of the prior art, the alloy of this invention still retains the ⁇ ' and S' coherent phases, which are a desirable microstructure for high temperature applications. Because of the unique Cu/Mg ratio for the alloy of this invention, ⁇ ' still maintains its coherency to the matrix even after it has been soaked at 600 °F for 100 hours. During soaking at 600 °F, ⁇ ' grew slightly in thickness but it did not coarsen, and still maintained a small diameter and coherency, which is critical for achieving high strength at elevated temperatures.
• FIG. 5 is an electron micrograph showing the transformation of the ⁇ ' and S' coherent phases, as observed in FIG 3, into the undesirable ⁇ and S noncoherent phases for the prior art alloys after they have been exposed to 600 °F for 100 hours.
• the ⁇ ' phase from other prior art alloys coarsens significantly and loses its coherency at elevated temperatures, thus resulting in a drastic loss in strength for elevated temperature applications.
• FIG. 2 is a diagram illustrating a non-coherent particle having no crystal structural relationship with the surrounding aluminum matrix atoms. Such an alloy has little or no practical application at elevated temperatures.
• Nickel improves the alloy tensile strength at elevated temperatures by reacting with aluminum to form the Al Ni 2 and Al 3 Ni compounds, which are stable metallurgical phases to resist the degradation effects from the long-term exposure to high temperature environments.
• Strontium is used to modify the Al-Si eutectic phase.
• the strength and ductility of Al-Si alloys having less than or equal to 12 wt% silicon are substantially improved with finer grains by using strontium as an Al-Si modifier.
• Phosphorus is used to modify the Silicon primary particle size when the silicon concentration is greater than 12 wt%, preferably 14 to 20. Effective modification is achieved at a very low additional level, but the range of recovered strontium and phosphorus of 0.001 to 0.1 wt.% is commonly used.
• the casting article In order for these strengthening mechanisms to function properly within the alloy, the casting article must have a unique combination of chemical composition and heat treatment history.
• the heat treatment is specifically designed to maximize the performance of the unique chemical composition.
• the exceptional performance of the alloy of the present invention is achieved by the combination of the following strengthening mechanisms through a unique heat treatment schedule.
• the heat treatment for the alloy of this invention was developed to maximize the formation of ⁇ ' and S' phases in the alloy (high volume fraction), to stabilize ⁇ ' phase at elevated temperature by controlling Cu/Mg ratio, and to maximize the formation of Al 3 (Ti, V, Zr) compounds for additional strengthening with mechanisms simultaneous addition of Ti, V, and Zr.
• the alloy of the present invention is processed using conventional gravity casting in the temperature range of about 1325 °F to 1450 °F, without the aid of external pressure, to achieve dramatic improvement in tensile strengths at 500 °F to 700 °F.
• pressure casting techniques such as squeeze casting.
• An article such as a cylinder head, engine block or a piston, is cast from the alloy, and the cast article is then solutionized at a temperature of 900 °F to 1000 °F for fifteen minutes to four hours.
• the purpose of the solutionizing step is to dissolve unwanted precipitates and reduce any segregation present in the alloy. For applications at temperatures from 500 °F to 700 °F the solutioning treatment may not be required.
• the cast article is advantageously quenched in a quenching medium, at a temperature within the range of 120 °F to 300°F, most preferably 170 °F to 250 °F.
• the most preferred quenching medium is water.
• the cast article is aged at a temperature of 425 °F to 485 °F for six to 12 hours.
• FIG 7 is a chart which illustrates the dramatic improvement in the ultimate tensile strength (UTS) at elevated temperatures for a cast article produced according to the present invention. It is a chart showing a comparison of an alloy according to the present invention with three well-known prior art alloys (332, 390 and 413). The chart compares the UTS (tested at 500 °F, 600 °F and 700 °F), after exposure of all test specimens to a temperature of 500 °F, 600 °F, 700 °F for 100 hours, respectively.
• UTS ultimate tensile strength
• the tensile strength of cast articles, prepared according to this invention is more than three times that of those prepared from the conventional eutectic 413.0 alloy, and more than four times that of those prepared from hypo-eutectic 332.0 alloy and the hyper-eutectic 390.0 alloy, when tested at 700 °F.
• the alloy of the present invention may be used in a bulk alloy form. It may also be used as an alloy matrix for the making of aluminum metal matrix composites (MMC).
• MMC aluminum metal matrix composites
• Such composites comprise the aluminum alloy of the present invention as a matrix containing a filler material, which is in the form of particles, whiskers, chopped fibers and continuous fibers.
• a filler material which is in the form of particles, whiskers, chopped fibers and continuous fibers.
• One of the most popular ways to produce an MMC is to mechanically mix and stir various ceramic materials in the form of small particles or whiskers into a molten aluminum alloy. This process has been called compo-casting or stir-casting of metal composite. In stir-casting techniques, the approach involves mechanical mixing and stirring of the filler material into a molten metal bath.
• the equipment usually consists of a heated crucible containing molten aluminum alloy, with an electric motor that drives a paddle-style mixing impeller, that is submerged in the molten metal.
• the filler material is poured slowly into the crucible above the melt surface and at a controlled rate, to ensure smooth and continuous feed.
• the temperature is usually maintained below the liquidus temperature to keep the aluminum alloy in a semi-solid condition in order to enhance the mixing uniformity of the filler material.
• the mixing impeller rotates at moderate speeds, it generates a vortex that draws the reinforcement particles into the melt from the surface.
• the impeller is designed to create a high level of shear force, which helps to remove the adsorbed gases from the surface of the particles.
• the high shear also engulfs the particle in molten aluminum alloy, which promotes particle wetting in order to enhance the homogeneous distribution of the filler material within the MMC.
• the filler materials or reinforcement materials added into the aluminum MMC usually have minimum dimensions which are much greater than 500 nm, typically in the range of 1 to 20 microns.
• Suitable reinforcement materials for making aluminum metal matrix composite include common materials such as Silicon Carbide (SiC), Aluminum Oxide (AI 2 O 3 ), Boron Carbide (B 4 C), Boron Nitride (CN), Titanium Carbide (TiC), Yttrium Oxide (Y 2 O 3 ), Graphite, Diamond particles and mixtures thereof. These reinforcement materials are present in volume fractions up to about 60% by volume, and more preferably 5-35% by volume.

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